混合二甲苯系統(tǒng)熱力學(xué)平衡組成的計(jì)算

趙　巖，任冬梅，夏云生，李新華，包德才

(渤海大學(xué)化學(xué)化工學(xué)院，遼寧 錦州 121013)

?

有機(jī)化工與催化

混合二甲苯系統(tǒng)熱力學(xué)平衡組成的計(jì)算

趙巖*，任冬梅，夏云生，李新華，包德才

(渤海大學(xué)化學(xué)化工學(xué)院，遼寧 錦州 121013)

摘要：研究混合二甲苯模型中3種組分在不同溫度下的熱力學(xué)平衡組成，依據(jù)狀態(tài)函數(shù)特點(diǎn)，分別計(jì)算各平衡的標(biāo)準(zhǔn)摩爾反應(yīng)熵變、標(biāo)準(zhǔn)摩爾反應(yīng)焓變及標(biāo)準(zhǔn)摩爾吉布斯自由能變，進(jìn)而計(jì)算平衡組成。結(jié)果表明，在該混合二甲苯理想氣體模型中，反應(yīng)溫度由298.15 K升至1 023.15 K，混合二甲苯中3個(gè)平衡組成隨著反應(yīng)溫度升高呈如下變化規(guī)律：間二甲苯含量y(MX)由0.599 0逐漸降至0.496 3，降幅達(dá)17.15%；鄰二甲苯含量y(OX)由0.162 6逐漸升至0.275 9，升幅達(dá)69.68%；對(duì)二甲苯含量y(PX)由0.238 4緩慢升至0.242 6，而后逐漸降至0.227 7，最大值出現(xiàn)在423.15 K，最大降幅僅為6.14%，表明升高溫度不利于提高對(duì)二甲苯的平衡組成。

關(guān)鍵詞：化學(xué)熱力學(xué)；混合二甲苯；平衡組成；對(duì)二甲苯；異構(gòu)化

CLC number:TQ241.1+3;O643.12Document code: AArticle ID: 1008-1143(2016)05-0075-06

混合二甲苯是間二甲苯(MX)、鄰二甲苯(OX)和對(duì)二甲苯(PX)構(gòu)成的混合體系，如指定了某一溫度和壓力，則該系統(tǒng)的狀態(tài)函數(shù)不再變化，即此時(shí)系統(tǒng)處于熱力學(xué)動(dòng)態(tài)平衡，其組分含量是定值。當(dāng)混合二甲苯看作理想氣體或理想液體混合物時(shí)，可以對(duì)系統(tǒng)熱力學(xué)性質(zhì)進(jìn)行演繹推理，為深入開發(fā)混合二甲苯轉(zhuǎn)化工藝提供重要的熱力學(xué)基礎(chǔ)數(shù)據(jù)。工業(yè)合成對(duì)二甲苯的工藝(如甲苯歧化、烷基轉(zhuǎn)移和汽油裂解等)主要產(chǎn)物為混合二甲苯[1-3]，因此要設(shè)計(jì)高效分離設(shè)備，對(duì)生產(chǎn)操作(如冷凝、氣化、閃蒸、精餾、吸收、萃取、結(jié)晶和吸附)進(jìn)行優(yōu)化。本文對(duì)混合二甲苯系統(tǒng)熱力學(xué)平衡組成的計(jì)算進(jìn)行研究。

1模型建立

圖1　理想二甲苯混合系統(tǒng)模型Figure 1　Ideal model of xylene mixed system

2計(jì)算過(guò)程

計(jì)算平衡組成熱力學(xué)方程式及計(jì)算路線：

3結(jié)果與討論

表1　二甲苯的標(biāo)準(zhǔn)摩爾恒壓熱容與溫度的關(guān)系[4-5]

表2　不同反應(yīng)溫度下二甲苯平衡體系的標(biāo)準(zhǔn)摩爾反應(yīng)熵變(T)

表3　不同反應(yīng)溫度下二甲苯平衡體系的標(biāo)準(zhǔn)摩爾反應(yīng)焓變(T)

表4　不同反應(yīng)溫度下二甲苯平衡體系的標(biāo)準(zhǔn)摩爾反應(yīng)吉布斯自由能變(T)

從表4可以看出，反應(yīng)溫度低于673.15 K，僅有鄰二甲苯向?qū)Χ妆降漠悩?gòu)化反應(yīng)可以自發(fā)進(jìn)行，且隨著反應(yīng)溫度升高，自發(fā)進(jìn)行的趨勢(shì)越來(lái)越弱；反應(yīng)溫度高于673.15 K，上述3個(gè)異構(gòu)化反應(yīng)均不能自發(fā)進(jìn)行，且隨著反應(yīng)溫度升高，自發(fā)程度越來(lái)越難。

3.5平衡常數(shù)及平衡組成

幅為17.15%；鄰二甲苯含量隨著反應(yīng)溫度升高逐漸提高，增幅為69.68%；對(duì)二甲苯含量隨著反應(yīng)溫度升高先增后降，降幅為6.14%。從熱力學(xué)角度看，反應(yīng)溫度變化對(duì)二甲苯含量影響較小，而升高溫度更有利于間二甲苯向鄰二甲苯異構(gòu)。因此，若要提高對(duì)二甲苯選擇性，需要從反應(yīng)動(dòng)力學(xué)著手[9-11]。選擇合適的擇形催化劑并適當(dāng)改性[12-23]以及設(shè)計(jì)新的分離工藝[24]或合成路線[25]均至關(guān)重要。

表5　不同反應(yīng)溫度下二甲苯平衡體系的氣相組成

4結(jié)論

(1) 通過(guò)對(duì)混合二甲苯熱力學(xué)平衡組成的計(jì)算可知，間二甲苯含量隨著反應(yīng)溫度升高逐漸降低，鄰二甲苯含量隨著反應(yīng)溫度升高逐漸提高，對(duì)二甲苯含量隨著反應(yīng)溫度升高先增加后降低，但降幅較小。升高溫度有利于促進(jìn)間二甲苯向鄰二甲苯異構(gòu)，而對(duì)二甲苯含量變化不大。若要提高對(duì)二甲苯選擇性，需從選擇催化劑、分離工藝和新合成路線入手。

(2) 混合二甲苯系統(tǒng)熱力學(xué)平衡組成的計(jì)算結(jié)果將為產(chǎn)品質(zhì)量控制提供至關(guān)重要的熱力學(xué)參數(shù)和多元相平衡數(shù)據(jù)，經(jīng)過(guò)對(duì)某些參數(shù)的適當(dāng)修正，該方法適用于其他理想混合體系的熱力學(xué)平衡計(jì)算。

參考文獻(xiàn):

[1]Chen N Y,Kaeding W W,Dwyer F G.Para-directed aromatic reactions over shape-selective molecular sieve zeolite catalysts[J].Journal of the American Chemical Society,1979,101(22):6783-6784.

[2]Paul B Weisz.Molecular shape selective catalysis[J].Pure and Applied Chemistry,1980,52:2091-2103.

[3]Kaeding W W,Chu C,Young B,et al.Shape-selective reactions with zeolite catalysts Ⅱ.Selective disproportionation of toluene to produce benzene and p-xylene[J].Journal of Catalysis,1981,69:392-398.

[4]John A Dean.Lange’s Handbook of Chemistry[M].United States of America:McGraw-Hill Book Company,1967.

[5]Robert C Weast.Handbook of Chemistry and Physics[M].70th ed.London:Wolfe Medical Publications Ltd.,1989.

[6]Rozanska X,van Santen R A,Hutschka F,et al.A periodic DFT study of intramolecular isomerization reactions of toluene and xylenes catalyzed by acidic mordenite[J].Journal of the American Chemical Society,2001,123(31):7655-7667.

[7]康承琳,龍軍,周震寰,等.二甲苯異構(gòu)化的反應(yīng)化學(xué)[J].石油學(xué)報(bào)(石油加工),2012,28(4):533-537.

Kang Chenglin,Long Jun,Zhou Zhenhuan,et al.Reaction chemistry of xylene isomerization[J].Acta Petrolei Sinica(Petroleum Processing Section),2012,28(4):533-537.

[8]李玲玲,聶小娃,宋春山,等.H-ZSM-5分子篩催化二甲苯異構(gòu)化的反應(yīng)機(jī)理[J].物理化學(xué)學(xué)報(bào),2013,29(4):754-762.

Li Lingling,Nie Xiaowa,Song Chunshan,et al.Isomerization mechanism of xylene catalyzed by H-ZSM-5 molecular sieve[J].Acta Physico-Chimica Sinica,2013,29(4):754-762.

[9]李玉光,胡軍.二甲苯在改性ZSM-5沸石上異構(gòu)化動(dòng)力學(xué)[J].化學(xué)物理學(xué)報(bào),1996,9(4):339-344.

Li Yuguang,Hu Jun.Kinetics of xylene isomerization on modified ZSM-5 zeolites[J].Chinese Journal of Chemical Physics,1996,9(4):339-344.

[10]李增和,劉秀英,馬麗景,等.HZSM-5催化劑上間二甲苯異構(gòu)化反應(yīng)動(dòng)力學(xué)研究[J].北京化工大學(xué)學(xué)報(bào),1997,24(4):67-70.

Li Zenghe,Liu Xiuying,Ma Lijing,et al.Reaction kinetics of m-xylene isomerization over zeolite ZSM-5[J].Journal of Beijing University of Chemical Technology,1997,24(4):67-70.

[11]陳金仙,張春,婁玥蕓,等.HZSM-5分子篩上間二甲苯異構(gòu)化反應(yīng)動(dòng)力學(xué)研究[J].南京工業(yè)大學(xué)學(xué)報(bào)(自然科學(xué)版),2012,34(3):66-69.

Chen Jinxian,Zhang Chun,Lou Yueyun,et al.Reaction kinetics simulation of m-xylene isomerization over HZSM-5 zeolite catalyst[J].Journal of Nanjing Universsity of Technology(Natural Science Edition),2012,34(3):66-69.

[12]Zhao Y,Wu H Y,Tan W,et al.Effect of metal modification of HZSM-5 on catalyst stability in the shape-selective methylation of toluene[J].Catalysis Today,2010,156(1/2):69-73.

[13]Zhao Y,Tan W,Wu H Y,et al.Effect of Pt on stability of nano-scale ZSM-5 catalyst for toluene alkylation with methanol into p-xylene[J].Catalysis Today,2011,160(1):179-183.

[14]Ahn J H,Kolvenbach R,Al-Khattaf S S,et al.Methanol usage in toluene methylation with medium and large pore zeolites[J].ACS Catalysis,2013,3:817-825.

[15]Lu P,Fei Z Y,Li L,et al.Effects of controlled SiO2deposition and phosphorus and nickel doping on surface acidity and diffusivity of medium and small sized HZSM-5 for para-selective alkylation of toluene by methanol[J].Applied Catalysis A:General,2013,453:302-309.

[16]Tan W,Liu M,Zhao Y,et al.Para-selective Methylation of toluene with methanol over nano-sized ZSM-5 catalysts:synergistic effects of surface modifications with SiO2,P2O5and MgO[J].Microporous and Mesoporous Materials,2014,196:18-30.

[17]Magdolna R M,Márton K,Szilvia K,et al.Transformation of ethylbenzene-m-xylene feed over MCM-22 zeoliteswith different acidities[J].Applied Catalysis A:General,2014,476:19-25.

[18]Zepeda T A,Pawelec B,Infantes-Molina A,et al.Ortho-xylene hydroisomerization under pressure on HMS-Ti mesoporous silica decorated with Ga2O3nanoparticles[J].Fuel,2015,158:405-415.

[19]Zepeda T A,Infantes-Molina A,Díaz de Leon J N,et al.Synthesis and characterization of Ga-modified Ti-HMS oxidematerials with varying Ga content[J].Journal of Molecular Catalysis A:Chemical,2015,397:26-35.

[20]Wu Y L,Li J C,Chai Y M,et al.Synergetic effect of H-ZSM-5/silicalite-1@Pt/Al2O3core-shell catalyst to enhance the selective hydrogenation of p-xylene[J].Journal of Membrane Science,2015,496:70-77.

[21]Ding W J,Li Hui,Pfeifer P,et al.Crystallite-pore network model of transport and reaction of multicomponent gas mixtures in polycrystalline microporous media[J].Chemical Engineering Journal,2014,254:545-558.

[22]Wu Q M,Wang X,Meng X J,et al.Organotemplate-free,seed-directed,and rapid synthesis of Al-rich zeolite MTT with improved catalytic performance in isomerization of m-xylene[J].Microporous and Mesoporous Materials,2014,186:106-112.

[23]Ahn J H,Kolvenbach R,Gutiérrez O Y,et al.Tailoring p-xylene selectivity in toluene methylation on medium pore-size zeolites[J].Microporous and Mesoporous Materials,2015,210:52-59.

[24]Gonalves J C,Rodrigues A E.Simulated moving bed reactor for p-xylene production:adsorbent and catalyst homogeneous mixture[J].Chemical Engineering Journal,2014,258:194-202.

[25]Zhang Y Y,Li Y F,Chen L,et al.A newcatalytic process for the synthesis of para-xylene through benzene methylation with CH3Br[J].Catalysis Communications,2014,54:6-10.

Calculation of thermodynamic equilibrium compositions of mixed xylenes

Zhao Yan*, Ren Dongmei, Xia Yunsheng, Li Xinhua, Bao Decai

(College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121013, Liaoning, China)

Abstract:The thermodynamic equilibriums of three components of mixed xylenes model under different temperatures were researched.Based on the characteristics of the state function,three thermodynamic functions (T) and (T) of three equilibriums components under different temperatures were calculated,respectively.And then the equilibrium compositions were calculated.The results showed that in the ideal gas model of mixed xylene,the change rules of the three equilibrium compositions with the increase of reaction temperatures from 298.15 K to 1 023.15 K were as follows:the y(MX)of m-xylene contents reduced gradually from 0.599 0 to 0.496 3 with the drop of 17.15%;the y(OX)of o-xylene contents enhanced gradually from 0.162 6 to 0.275 9 with the increase of 69.68%;the y(PX)of p-xylene contents increased slowly from 0.238 4 to 0.242 6,and then decreased gradually to 0.227 7.Its maximum of 0.242 6 at 423.15 K with the biggest drop of 6.14% was obtained,which indicated that increasing temperature was unfavorable to improving balance composition of p-xylene.

Key words:chemical thermodynamics; mixed xylenes; equilibrium composition; p-xylene; isomerization

收稿日期：2016-03-16

基金項(xiàng)目：渤海大學(xué)博士啟動(dòng)基金(BSQD201416，BSQD201417)資助項(xiàng)目；國(guó)家自然科學(xué)基金項(xiàng)目(21076026)；遼寧省教育廳項(xiàng)目(L2013430)

作者簡(jiǎn)介：趙巖，1976年生，男，遼寧省錦州市人，博士，講師，研究方向?yàn)槎嘞啻呋?/p>

doi:10.3969/j.issn.1008-1143.2016.05.015 10.3969/j.issn.1008-1143.2016.05.015

中圖分類號(hào)：TQ241.1+3;O643.12

文獻(xiàn)標(biāo)識(shí)碼：A

文章編號(hào)：1008-1143(2016)05-0075-06

通訊聯(lián)系人：趙巖。